The present invention relates to sewing machine speed control systems, and in particular to a digital sewing machine speed control system in which machine speed is rapidly controlled in response to a sudden occurrence of load change.
Our corresponding U.S. patent application (later filled) discloses a digital speed control system for sewing machines which comprises a digital pedal depression sensor and a programmable frequency divider which divides the frequency of speed indicative pulses by an integral multiple that is a function of a digital signal supplied from the depression sensor. A period detector is provided to measure the interval between successively generated pulses from the frequency divider. The sewing machine is driven by a motor which includes magnetically operated clutch and brake means which are respectively energized by a pulse having a duration that is a function of the measured interval. The measured interval is represented by binary data which is applied to a digital arithmetic circuit that translates the input binary data into period data according to a predetermined transfer function describing the relationship between the measured interval and a period in which the clutch or brake means is energized. The digital pedal depression sensor includes a digital coding plate which is movable with the foot pedal with respect to sensing device to generate position signals.
When the frequency division ratio of the programmable divider is changed in response to a change in pedal position in the speed increase direction, the pulse interval of the frequency divider is multiplied according to the change in frequency division ratio. The arithmetic circuit provides a period data which increases the duty cycle of the clutch means. The sewing machine speed increases until the measured pulse interval reaches the previous value. When the sewing machine is stabilized at the changed frequency division ratio, the pulse interval of the frequency divider is maintained constant.
It is of particular importance that the speed control system for sewing machines respond quickly to a sudden change in sewing load so that the sewing machine quickly returns to the nominal speed, which is thus maintained constant under varying load.
The copending United States application is unsatisfactory in respect of the speed response characteristics because the transfer function of the arithmetic circuit takes into account only one input variable, i.e. the pulse interval of the frequency divider.